Process for manufacture of strong dinitrogen tetroxide



A. W. YODIS Nov. Z9, i955 PROCESS FOR MANUFACTURE OF STRONG DINITROGEN TETROXIDE Filed Feb. ll, 1955 TO 2N EY.

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United States Patent PRCESS FUR MANUFACTURE F STRONG DINI'I'ROGEN T ETROXIDE Anthony W. Yodis, Claym'ont, Del., assigner to` Allied Chemical'& Dye Corporation, New York, N. Y., a corporation of New York ApplicationFebruaryH, 1953, Serial No. 336,3695 sciaims. (ci. 23157) This invention relatesto theproduction of dinitrogen tetroxide and 'more particularly refers to anew and im,- proved! process for converting sulfur and nitric acid, into high yields of dinitrogen tetroxide.

Various methods have been proposed in recent years for theA manufacture of N204. These methods depend on the -use of heat to crack 'nitric acid in concentrations in excess of 95%v nitric or the absorption of N02 from ammonia oxidation units in various desiccants and, their subsequent liberation by heat and absorption. Cracking of nitricaeid is not an economical method particularly due to the requirement for materials of construction which will withstand the high temperatures of reaction.. Likewise the desiccant absorption method involves high fixed capital expenditure and operating cost.

Many years ago the suggestion was made to prepare N02 (alternatively referred to' as N204) by reacting nitric acidL with sulfur. In the conversion of nitric acid and sulfur for production of N204, low yields of the desired prod uct areobtaineddue to the incomplete conversion of the reactants and` concomitant production of substantial quantities of lay-products vwhich generally are nitrosyl sulfuric acid. and its anhydride,v N0, N203, and Water. The dinitrogen tetroxide produced is usually of inferior quality, green in color and contaminated with by-products difficult to remove. Obviously, such process vhad no practical utilityy and was never installed commercially. n

One object of the present invention is to provide an efficient continuousprocess for converting sulfur and nitric acid into dinitrogen tetroxide.

A Another object o f thisinvention is toprovidel aprocess for producing high yieldsV of high quality dinitrogen tetroxide. v y t Further objects andadvantages will be apparent from the `followingdescription and accompanying drawing,

4ln 'accordancel with the, present invention, dinitrogen tetroxide. maybeproduced by reacting nitric acid,y preferablyin-exess of 80% HN03 concentration and more desirablyy in excessA of 95% HNOS concentratiomy and ele.- mental sulfur in a first reaction none preferably maintained at lapternperature within the range, of 100 C., desirably 119 C.,to. a-temperature below the meltingpoint of sulfur; thereby producing `as primary- *reaction products dinitrogentetroxide and sulfuric ,acid together with small amounts of nitrosyl sulfuric: acid, its anhydride, NO and `N203, releasing vapors. from the first zone comprising primarily N204, yHNOa, H together with minor amounts of nitrosyl sulfuric, acid anhydrideand lower oxides. of nitrogen 'such as N0 and N203, passing said vapors upvvardlyy through` a second reacting and separating zone countercurrentvfto and in intimate contact` with cooler concentrated nitric acid to condense substantially all the vapor constituents havingfa` boiling point above 86 C.

and simultaneously convert at least apart of the lower oxides of nitrogen and nitrosyll sulfuric acid anhydride to additional dinitrogen tetroxide, returning the liquid con,- dens'ate from the second zone to the first Zone, releasing a" vapor mixture comprising substantially anhydrous 2,725,280 Patented- Nw. 29, 1255 ice HNOa and N204 from the second zone, coolingsaidy vapor mixture to effect condensation` and separation ,ofV substantially anhydrous HN03, returningi said anhydrous HNOS condensate to the second zone'for intimate contact'with vapors entering the. second Zone from the iirst zone, and condensing and, collecting the N204 vapors separatedl from the mixture of HNOaand N204 leaving the second zone. j While- I do notwish topredicate my invention on any theoryV of the mechanism of the reaction, the lfollowing explanation may aid in a= better` understanding of the invention.

The reaction betweeny nitric acid and sulfur is quit complex and not thoroughly understood. One of the'primary reactions involved may be illustrated bythe following equation:

6HNO3+S 3N2'O4-i-H2SO44l-2H2O Another strong competing reaction occurring simultaneously may be illustrated by the following equation:

Otherrside reactions, the mechanism of which is not fully understood, involve the production of- NO, N203, N205, HN0SO4 and (N0)2Sz0'z.I

From theforegoing it will be` evident that low yields and poor quality of the desired product dinitrogen tetrox'ide` are produced4 by reaction of sulfur and nitric aeiddue in p art to the formation oflarge quantities of by-profductsy resulting from competing reactions. ToV illus,- trate more specicaily, when nitric acid was added to sulfur, as suggested by the art, there was produced a vapor product composed of not more than about N204,V over 5%, lsulfur compounds,I several percent lower oxides of nitrogen, down to 15% vor more HNO3 and' water. The residual acid` contained over 40% unreacted HNOS, over 15% nitrosyl sulfuric acid, about 20% sulfuric acid and abouty 20% water and other by-products. Thus, the yield of jdinitrogen tetroxide produced was less than 60% based onf` the feed l nitric acid and the resulting dinitrogen tetr'oxide product was ycontaminated with a large amount of impurities whichare most diiicult to separate.

One'l of the 'by-products formed in large quantities, approximately 20%, during reaction of nitric acid and sulfur when carried out in ya conventional manner is nitrosyl sulfuric acid. In the course of my investigation I noted that the nitrosyl sulfuric acid appearedy in the N204 vapor product primarily in the form of fnitrosyl s ulfuricacid anhydride, i. e.`nitrosyly sulfuric acid was substantially nonvolatile under the conditions ofy reaction and the anhydride was relatively quite volatile. v I discovered that the maintenance of al copious amount of moisture in the reaction zone retarded the formation of nitrosyl sulfuric acid anhydride and also had the elect of substantilally,reducingl the amount of nitrosyl sulfuric acid in the residual acid; I found favorable moisture conditions to minimize the formationfof nitrosyl sulfuric acid and its anhydride could be attained by preventing discharge of water vapor from the system and retaining the water of reaction in the reaction zone. Retentiony of water in the reaction zone is accomplished bycondensing water vapor leavingl the reaction zone to prevent it from discharging fromf'the( system and returning such water condensate to the reaction, zone "thereby retaining a high moisture content at all times incontact with the. reactants. .0fv course, water from an external source may be added. to the reaction mixture but this procedure is not entirely satis; factory'because the products are diluted and the capacity is reduced." `I have further found that the quantitiesof Aother by-products',v namely, the lower oxides of nitrogen andvr to'some extent nitrosyl sulfuric acid may be conl verted to the desired dinitrogen tetroxide product by subjectngfthe vapor product? resulting from the reaction of nitric acid and sulfur, in intimate contact with substantially anhydrous nitric acid in a second zone. Concomitantly, I effect condensation of the high boiling contaminants in the second zone.

The accompanying drawing is a diagrammatic flow sheet illustrating the process of the present invention.

The charging materials consisting of elemental sulfur in iinely divided form and nitric acid are introduced through lines 1 and 2 respectively into converter still 3 which may be an empty enclosed chamber constructed of an acid resistant metal such as duriron. Converter still 3 is desirably equipped with a stirrer 4 to maintain the contents in constant agitation. Although the reaction is exothermic, under the conditions of operation a small amount of additional heat may be required which can be provided by conventional tantalum heater 5. A convenient method of initiating the reaction is to retain residual acid from a previous run in converter still 3, heat the mixture to the desired reaction temperature and then continuously feed nitric acid and sulfur through lines 1 and 2 to the heel of residual acid in still 3. Another method of initiating the reaction would be to dispose in converter still 3 a mixture of elemental sulfur and sulfuric acid of about 75% concentration and to this mixture add nitric acid to start the reaction and thereafter feed nitric acid and sulfur through lines 1 and 2. The reaction mixture in still 3 is maintained at a temperature within the range of about 100 C. to a temperature just below the melting point of sulfur. Sulfur has a melting point of about 115 C., the melting point varying a few degrees in either direction of 115 C. depending upon the form of sulfur, i. e. whether the sulfur is in a rhombic, monoclinic or amorphous form or a mixture thereof. Although the operation may be carried out at temperatures up to 135-140 C., at temperatures substantially in excess of the melting point of sulfur there is a tendency for dinitrogen tetroxide to decompose; more important, if the sulfur ,is in molten condition in the reaction mixture it would make continuous operation diiiicult in that the molten sulfur would block or leak through the pores of filter 6 disposed near the top of the reaction mixture for the purpose of separating residual acid withdrawn through line 7. Also, when the sulfur is in the molten condition the rate of reaction is considerably reduced due to the smaller area of sulfur surface in contact with the reacting acid. Filter 6 may be any suitable filter for separating liquid from solid and for examplemay be constructed of porous fused alumina. At temperatures below 100 C. the reaction requires an unduly long time. The preferred temperature range for carrying out the reaction is just a few degrees below the melting point of sulfur, or stated another way, from about 110 C. to near the melting point of sulfur. The operation may conveniently be carried out at atmospheric pressures; superatmosphcric or subatmospherie pressures are unnecessary.

The relative proportion of sulfur and nitric acid fed into converter still 3 is not critical provided there is always asuicient excess of elemental sulfur for reaction with nitric acid in converter still 3. In general the operation may be carried out conveniently by introducing sulfur and HNOs in the proportion of 1 to 12.6 parts by weight. f greater significance to the operation, is the concentration of nitric acid introduced as feed. Although the operation will work well with a nitric acid concentration feed in excess of 80%, preferably the nitric acid feed should be in excess of 95%. The reason for this is that with the lower concentrations of nitric acid feed there is produced a more dilute residual acid, i. e. a residual acid containing more water making more diicult and expensive the recovery of nitric acid and sulfuric acid therefrom.

The process of the present invention is particularly adapted for continuous operation in which procedure elemental sulfur and HNOa are continuously fed into converter still 3, vapor product continuously released from the top of still 3 through vapor line 8 and residual acid continuously withdrawn through line 7 in an amount suicient to maintain a liquid level in still 3. Although not a preferred method of operation, the process may be carried out in semi-continuous manner with continuous feed of sulfur through line 1 and HNOs through line 2, and discharge of vapors through line 8 but without any withdrawal of residual acid through line 7 during the operation. After the liquid content in still 3 has built up to too high a level so .as not to provide adequate vapor space for the release of vapors, the run is terminated and the still contents in whole or part discharged and a new run started. During the operation, whether continuous or semi-continuous, intimate contact of the reactants in the form of a slurry of sulfur in residual acid is assured by constant agitation with stirrer 4. The vapor released from the top of converter still 3 through vapor line 8 is a mixture containing predominantly N204 and -IN03 together with appreciable amounts of H20, and smaller amounts of nitrosyl sulfuric acid anhydride and lower oxides of nitrogen. The vapors are introduced into reaction and separating tower 9 at a point near the bottom thereof, which tower is constructed of corrosive-resistant materials, for example, the shell of the tower may be made of duriron and the tower packed with sections of ceramic packing of an acid-resistant type. Vapor line 8 and liquid reflux return line 11 should also be constructed of a non-corrosive material such as duriron. The vapors entering through line 8 pass upwardly through the packing in tower 9 countercurrent and in intimate contact with liquid, substantially anhydrous HNOS, entering the top of tower 9 through line 12. The anhydrous HNOa reacts with at leasty part of the lower oxides of nitrogen and nitrosyl sulfuric acid anhydride converting these impurities to additional dinitrogen tetroxide. The amount and temperature of the HNOa entering through line 12 should be sufficient to condense substantially all constituents of the vapor having a boiling point higher than the HNOa. This may be readily determined by the conventional method of analyzing a sample of the vapors released from the top of tower 9 through line 13. Ordinarily a top tower temperature of 86 C. when operating at about atmospheric pressure will control the vapor composition through line 13 to permit the escape of a vapor mixture consisting of N204. and HN03 with a minor amount of other constituents. Reflux condensate composed primarily of H2O and HNOS, and smaller amount of nitrosyl sulfuric acid, H2804 and lower oxides of nitrogen ows by gravity down through reflux line 11 into converter still 3. The apparatus in the drawing shows converter still 3 and tower 9 connected by individual vapor line 8 and reflux line 11. For smaller installations, it may be more economical to construct the apparatus with tower 9 directly superimposed on an opening into the top of converter still 3. In such construction liquid reflux condensate from the bottom of the tower will fall down directly into converter still 3 and vapor from still 3 would rise directly into the bottom of tower 9.

The N204 and HNOs vapor from line 13 enters hot condenser 14 maintained at a temperature slightly above the boiling point (22 C.) of N204, i. e. a temperature of about 24 C. to effect condensation of the HNOs which is returned via line 12 to the top of tower 9.

Control of the temperature of hot condenser 14 is ob-v tained by indirect contact with cooling water entering and leaving through lines 15 and 16 respectively. p

Uncondensed vapors containing over N204 pass from hot condenser 14 through line 17 into cold condenser 18 where it is cooled by water entering and leaving through lines 19 and 21 to a temperature of about 10 C. to condense N204, and the liquid N204` then passes through line 22 and collects in receiver 23. Uncondensed gases are released from the top of receiver 23 through line 24. The liquid condensate product, directed to storage through line 25, contains over 95%, gener- 2nite-.aso

chemical-reactions'. Substantial amounts of HNOs` lcould,

be tolerated in` N202L and in fact-:mixturesof HNOs and N2O4.-are frequently employed in-chemical reactions.

Residualacid discharging from.v still 3 through `line 7, whenfrun--in' a. continuousmanner, contains. about 53% H2SO4,-about14% HN03, about 6% HNOSOg-about 25%- H20 and about 2% other by-products.. The composition of -residual acid is significant'. with respect. to `the lowpercentage of.v HNOS..J and `the high percentage of H2S04; The smalltamounttof -HNOs in the residual acid indicates-high` conversion' ofyHNOa4 fed. into thesystem. Asfaqmatter of fact,.theV yield. of dinitrogenftetroxide obtainedv by myfprocessbased on the amount. of HNOa chargedeisin excessfof75%, andi in some of myy operations I-have obtained-a yield-of over 90%.. The high concentration of HzSOiin-V, the residual acid lends itself tostheseparationv of,l HNO3 and a fairly. concentrated H2SO4-asr will nowbe described.A

Residual acid is introduced-through line 7 into the bottom of afpackedrr acid-resistantcolumn` 26. Steam entering-through-'line 27-is passed upwardly through the column?-thereby stripping HNO3 from-the residualacidE entering through line 7. Concentrated HNOS ris-released; from the top of column 26 through line 28, condensed in condenser129.' and.v thendirected througlr-linef 31 to receiver 32. Uncondensed gases are-vented through line 33 at the top of receiver 32. A portion of the concentrated nitric acid condensate is withdrawn from receiver 32j`and. forced by'pumpv 34-through` line "35 into the top of" column v26:-to control' the ytemperature at thef'top of column- 26:V to* permit'thereleaseof` concentrated nitric acid vapor through line 28. Concentrated nitric acid generally in-excess of 95% H-N03 -is discharged through line 36' and may be sent'to storage or recycled'to converter still 3 for'furth'erreactionfwith elemental sulfur. Dilutez'sulfuric acid, generally'about-66.% H2804. collecting, in the-'bottom of column 26,'1 isy discharged through" line 3'7v and if desired may be further'concentratedinfaf conventional manner to strong sulfuric acid. Although small in amount, I have found that the nitrosyl sulfuric acid contained in the residual acid is decomposed into sulfuric acid and nitric acid in column 26.

From the foregoing it will be apparent that my twozone conversion method under the conditions set forth, accomplishes highly eicient conversion of sulfur and nitric acid into dinitrogen tetroxide with a minor amount of undesirable compounds.

The following example illustrates the present invention.

A vessel equipped with a heating element and agitator and initially charged with a heel of residual acid from a previous run is heated to a temperature of 112 C. and maintained at that temperature by means of the heating element. Into the vessel are fed elemental sulfur and 97.2% HNOa at the rate of 19.23 pounds and 248.6 pounds per hour respectively. Vapors from the vessel are fed to the bottom of a vertical packed tower wherein they pass upwardly in intimate contact with down-flowing substantially anhydrous l-INOa. Reflux condensate collecting in the bottom of the tower is returned to the vessel. Vapors from the top of the tower are cooled to a temperature of 25 C. by indirect heat exchange with warm water in a hot condenser thereby condensing HNOs which is returned to the top of the tower for further contact with vapors entering the tower from the reaction vessel. The uncondensed vapors are cooled to a temperature of C. and the condensate collected as a product of the process. This product is produced at the rate of 164.5 pounds per hour and contains 96.5% N204, 2.2% HNO3,

is: withdrawn from. the. reactionU vessel. at. the rate fof.

101.17 ypounds perfhouryandvhas the. following composi-y tion-t. 53.19%"1-1'2505, 14.12%'. H-NOa, 61.1%-v I]I`.O,SO4,.4 and 26.59% H20.. The. yield. of dinitrogen. tetroxidey basedonfthe nitric acid charged is.90%

Although certain preferred 'embodimentsl of.` the, inventionhave been. disclosed` forpurpose, of.I illustration it'fwill` be evident that various changes and" modifications.. may. be made thereinf without departing from thel scope. andv spirit. of the invention..

`1.y A processtfor. the manufacture of dinitrogentetroxide,'v

' whichcomprisesreacting.. elemental sulfurv and nitric, acid.

thereby producing as primaryreactionproducts dinitrogen. tetroxide and. 'sulfuric acid, together. with nitrosyl sulfuric acid, nitrosyl sulfuric acid. anhydride.lower oxides of`v nitrogenand'HzOy as by products, `releasingvapors evolved. from said reaction comprising primarily.N204;y HNOsQf H20, and small: amounts. ofy nitrosyl. sulfuric anhydride and.v lower oxides of-nitrogen, cooling' said. vaporsto effect condensation:` of thev H2O, andfreturningsaid. condensate; to the mixture of elemental sulffur-andnit'ricacid under-- going. react-ion.

2. A process. for the4 manufacture of dinitrogen-tetro. ide f whichA comprises.- reacting, nitric acid 'and elemental1 sulfur. in a first. reaction zone. thereby producingy as primary. reaction products dinitrogen-tetroxide and sulfuricl acid together with-nitrosyl sulfuric. acid,A its-anhydride.;V lowerioxidesfof nitrogen andwater, as by-products, releasing vaporsafrom` the. first reaction zone comprisingprif' marily.- N2'O'4, HNOa.. H2O,v and smallI amounts. off nitrosyl..

sulfuric. acid anhydride. andlower oxides of nitrogen,. passing; said vapors upwardly through a second'reacting.V and separating zone'countercurrentpto and; in intimate contact with;y cooler concentratednitric acid to.- condense. substantially all the vapor constituentshaving laboilingpoint. above 8 6? C. and simultaneously.converting at-,least'f part of. the lower oxides-of` nitrogenl and=nitrosyl sulfuricaidqanhydride to dinitrogen.. tetroxide, and. ret-urningrthe:v liquidrcondensatefrom the second zone to the first zone 3... A. process. for. the.v manufacture. of. dinitrugen` tetroxiide 'whichl comprises reacting. nitric' acid,- and elemental@- sulfur.- in ay first reaction zone. thereby, producing-as p ri`-, maryf reaction-- products. dinitrogen tetroxide'v and. sulfuric. acid together; with nitrosyl sulfuric acid,v its; anhydride,` lower oxides, ofLni-trogemfand. water asbyproducts;` re.- leasing vapors from the rst reaction zone comprising primarily N204, HN03, H20, and small amounts of nitrosyl sulfuric acid anhydride and lower oxides of nitrogen, passing said vapors upwardly through a second reaction and separating Yzone countercurrent to and in intimate Contact with cooler concentrated nitric acid to condense substantially all the vapor constituents having a boiling point above 86 C. and simultaneously converting at least a part of the lower oxides of nitrogen and nitrosyl sulfuric acid anhydride to dinitrogen tetroxide, returning liquid condensate from the second zone to the first zone, releasing a vapor mixture comprising substantially anhydrous HNOs and N204 from the second zone, cooling said vapor mixture to effect condensation and separation of substantially anhydrous HNOa, returning said anhydrous HNOS condensate to the second zone for intimate contact with vapors entering the second zone from the first zone and condensing and collecting the N204 vapors separated from the mixture of HNO: and N204 leaving the second zone.

4. A process for the manufacture of dinitrogen tetroxide which comprises reacting nitric acid having a concentration in excess of HNOa and elemental sulfur in a first reaction zone maintained at a temperature between C. to a temperature below the melting point of sulfur, thereby producing as primary reaction products dinitrogen tetroxide and sulfuric acid together with nitrosyl sulfuric acid, its anhydride, loweroxides of nitrogen, and water as by-products, releasing vapors from the first raction zone comprising primarily N204, HNOa, H20, and'srnall amounts of nitrosyl sulfuric acid anhydride and lower oxides of nitrogen, passing said vapors upwardly through a second reacting and separating zone counter current to and in intimate contact with cooler concentrated nitric acid to condense substantially all the vapor constituentsv having a boiling point above 86 C. and simultaneously converting at least apart of the lower oxides of nitrogen and nitrosyl sulfuric acid anhydride to dinitrogen tetroxide, returning liquid condensate from the second zone to the first zone, releasing a vapor mixture comprising substantially anhydrous HNOa and N204 from the second zone, cooling said vapor mixture to effect condensation and separation of substantially anhydrous HNOa, returning said anhydrous HNOa condensate to the second zone for intimate contact with vapors entering the second z one from the first zone and condensing and collecting N204 vapors separated from the mixture of HNOs and N204 leaving the second zone.

5. A process for the manufacture of dinitrogentetroxide which comprises reacting nitric acid having a concentration in excess of-95% HNO3 and elemental sulfur in a first reaction zone maintained at a temperature between 110 C. to a temperature below the melting point of sulfur thereby producing as primary reaction products dinitrogen tetroxide and sulfuric acid together with nitrosyl sulfuric acid, `its anhydride, lower oxides of nitrogen, and water as by-products,v releasing vapors from the first reaction zone', comprising primarily N204, HNOs, H20, and small amounts of nitrosyl sulfuric acid anhydride and lower oxides of nitrogen, passing said vapors upwardly through al second reacting and 4separating zone countercurrent to and in intimate contact with cooler concentrated nitric acid to condense substantially all the vapor constituents having a boiling point above 86 C. and simultaneously converting at least a part of the lower oxides of nitrogen and nitrosyl sulfuric acid anhydride to dinitrogen tetroxide, returning liquid condensate from the second zone to the irst zone, releasing avapor mixture comprising substantiallyianhydrous HNOS and N204 from the second zone, cooling said vapor mixture to effect condensation and separation of substantially anhydrous HNOa, returning said anhydrous HNOs condensate to the'second zone for intimate contact with vapors entering the second zone from the first zone and condensing and collecting the N204 vapors separated from the mixture of HNOa and N204 leaving the second zone.

6. A process for the manufacture of dinitrogen tetroxide which comprises continuously introducing nitric acid having a` concentration in excess of HNOS and elemental sulfur in a first reaction zone maintained atv a temperature between C. to a temperature below the melting point vof sulfur, thereby producing as primary reaction products dinitrogen tetroxide and sulfuric acid together with nitrosyl sulfuric acid, its anhydride, lower oxides of nitrogen, and water as by-products, continuously releasing vapors from the first reaction zone comprising primarily N204, HNOs, H20, and small amounts of nitrosyl sulfuric acid anhydride and lower oxides of nitrogen, continuously passing said vapors upwardly through a second reacting and separating zone countercurrent to and in intimate contact with cooler concentrated nitric acid to condense substantially all the vapor constituents having a boiling point above 86 C. and simultaneously converting at least a part of the lower oxides of nitrogen and nitrosyl sulfuric acid anhydride to dinitrogen tetroxide, continuously returning liquid condensate from the second zone to the first zone, continuously releasing a vapor mixture comprising substantially anhydrous HNOa and N204 from the second zone, continuously cooling said vapor mixture to effect condensation and separation of substantially anhydrous HN03, continuously returning said anhydrous HNOa condensate to the second zone for intimate contact with vapors entering the second zone from the iirst zone, condensing and collecting the N204 vapors separated from the mixture of HNOa and N204 leaving the second zone, and continuously discharging residual acid from the first zone,

References Cited in the le of this patent UNITED STATES PATENTS .1,590,043 Leutz June 22, 1926 1,912,832 Fairlie June 6, 1933 2,053,834 Kacharo et al Sept. 8, 1936 2,450,105 Batchelder et al Sept. 28, 1948 FOREIGN PATENTS 515,429 Great Britain Dec. 5, 1939 OTHER REFERENCES lMellor: Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. 8, page 585, Longmans. 

1. A PROCESS FOR THE MANUFACTURE OF DINITROGEN TETROXIDE WHICH COMPRISES REACTING ELEMENTAL SULFUR AND NITRIC ACID THEREBY PRODUCING AS PRIMARY REACTION PRODUCTS DINITROGEN TETROXIDE AND SULFURIC ACID TOGETHER WITH NITROSYL SULFURIC ACID, NITOSYL SULFURIC ACID ANHYDRIDE, LOWER OXIDES OF NITROGEN AND H2O AS BY PRODUCTS, RELEASING VAPORS EVOLVED FROM SAID REACTION COMPRISING PRIMARILY N204, HNO3, H2O, AND SMALL AMOUNTS OF NITROSYL SULFURIC ANHYDRIDE AND LOWER OXIDES OF NITROGEN, COOLING SAID VAPORS T EFFECT CONDENSATION OF THE H2O, AND RETURNING SAID CONDENSATE TO THE MIXTURE OF ELEMENTAL SULFUR AND NITRIC ACID UNDERGOING REACTION. 